Pulse oximeters are commonly used in hospitals and other patient-care facilities for monitoring the blood oxygen level of a patient in a non-invasive, continuous manner. The basis of operation of these instruments is the fact that blood absorbs red (R) and infrared (IR) light in different amounts depending on the level of oxygen in the blood (SpO.sub.2) and that a known relationship exists between the ratio of red to infrared light (R/IR) and blood oxygen level (SpO.sub.2).
Thus, a pulse oximeter functions by detecting the amount of red and infrared light transmitted through a part of the body, usually a finger, to establish the R/IR ratio. It then compares this ratio with an internally stored database giving the relationship between R/IR ratios and SpO.sub.2 levels, determines the SpO.sub.2 level for the detected ratio, and displays the SpO.sub.2 value. Pulse rate and other parameters may also be detected and displayed.
The popular pulse oximeters provide an optical sensor, typically called a probe, which is an alligator- or clamshell-clip that clamps on the index finger of a patient. One jaw of the probe clip contains red and infrared light-emitting diodes (LEDs), and the other jaw contains a light detector such as a photodiode. The probe cable is connected to the main unit or instrument of the oximeter. Certain of the conductors in the cable connect the LEDs to a driver circuit in the instrument which produces a signal to activate the LEDs, and other conductors connect the photodiode to an amplifier for amplifying the small signal generated by the photodiode when light is transmitted through a finger and for transmitting it to the signal processor of the oximeter.
Like any instrument for monitoring physiological functions, a pulse oximeter needs to be tested on a regular basis to determine if it is providing accurate readings. Although test equipment has been developed for testing the accuracy of pulse oximeters, such equipment has been based on a particular testing philosophy which imposes certain undesirable limitations on the test results. This philosophy is to provide a simulated finger, clamp this "finger" in the oximeter probe, operate the oximeter to test the simulated finger just as if it were a real finger, and determine if the display provides a reading consistent with the simulation.
An example of this testing approach is disclosed in the Merrick et al. U.S. Pat. No. 5,348,005. In this patented device, the artificial finger is a long, narrow, finger-shaped object made of steel and having slots on opposite sides respectively receiving an LED bar and a pair of photodiode detectors. To test an oximeter, the Merrick et al. artificial finger is placed in the oximeter probe, and the oximeter is operated just as if it were monitoring a human subject. That is, the red and IR LEDs of the oximeter emit flashes of light that are detected in the "finger" by the photodiodes which convert the light pulses into electrical pulses. These pulses are modulated in the tester to provide signals which simulate the signals that would be developed from a human finger. The modulated electrical signals are then transmitted to the LEDs in the artificial finger which converts them to light flashes that activate the photodiode in the oximeter probe. Thus, the artificial finger is an optical interface between the oximeter under test and the test equipment.
The described testing approach is certainly the straightforward approach in that it simply replaces a real finger with an artificial finger and allows the oximeter to be used for the test in a manner similar to its use in monitoring a patient. The only difference appears to be the use of an electro-optical finger instead of a real finger. Such an approach, however, has not proved to be fully effective since the reliability of the testing can be compromised or indeterminate and desired testing flexibility is lacking, as hereinafter explained.
First, the testing accuracy of an oximeter tester using an optical interface, as described above, depends on proper placement of the simulated finger in the probe to allow the optical elements to interact properly. With a human finger, exact placement of the finger in the probe is not critical since the entire circumference of a finger presents human tissue to the LEDs. With an artificial finger, however, the photodiodes and the LEDs in the artificial finger must be placed exactly opposite the LEDs and photodiode, respectively, of the probe or else the light flashes will not be detected by the photodiodes. Thus, if this placement is not exact, inaccurate readings will occur.
Before discussing other deficiencies of oximeter test equipment employing an optical interface, it is important initially to understand the construction of commonly used oximeter cables and their normal use in a hospital. The cable referred to in one popular brand of oximeter is actually two cables, namely, a preamp cable and a probe cable. Although interconnected, these two cables are of different construction; the preamp cable is relatively strong and durable, whereas the probe cable is not as strong or durable. The preamp cable is a thicker, somewhat flexible, heavy duty cable, usually about 12 feet long, whereas the probe cable is a thinner, very flexible, lightweight cable, usually about 3 feet long. Whether or not the oximeter has a preamp cable, extension cables (also referred to as patient cables) are often provided and for the purpose of the present invention are equivalent to a preamp cable. In referring to the preamp cable hereinafter, it will be understood that such an extension cable is included.
The preamp cable is normally attached to and kept with its oximeter instrument as the latter is moved about the hospital, whereas the probe cable is normally kept separate from the oximeter instrument and is not associated with any particular instrument since it can be attached to any instrument of the same manufacture. The probe cable may even be a disposable item.
In a hospital room, the preamp cable is long enough to serve as an extension cord so that the oximeter instrument can be placed in a convenient position and the preamp cable stretched to the bedside of the patient. In contrast, the probe cable is clamped to the bed or bedclothes of the patient's bed and coupled to the preamp cable. The probe cable is made light and flexible so that it moves easily and folds as the patient moves about and is less of an annoyance to the patient. Thus, probe cables can easily fall on the floor by the bedside and be subjected to abuse. Moreover, they are normally folded, wound, or otherwise compressed and stored in a drawer, sometimes rather haphazardly.
Because of the described construction, use and treatment of probe cables and their probes, they are very vulnerable to damage and defects. Field studies with hospital biomedical technicians have shown that the majority of oximeter defects have been probe defects and that these defects are caused by frayed probe cable wires, that is, breaks or shorts, many of which occur intermittently. Compounding this problem is the fact that as indicated above, specific oximeter probes are not paired with specific oximeter instruments, that is, the probes are interchangeable among different oximeters and as stated above, the entire probe including the cable and the clip may be disposable.
Using the above described testing approach with an optical interface, i.e., inserting an artificial finger in the clamshell probe of the pulse oximeter, the testing device performs the test through the probe cable, that is, with the probe cable connected to the preamp cable, just as if the oximeter were being used on a human patient. If there is a defect in the probe cable, this testing approach may miss it or fail to isolate it.
For example, if the probe cable has frayed wires, front end circuitry in the oximeter may filter out the noise spikes caused by the fraying, and the display will indicate normal functioning, whereas in fact the probe cable is damaged and may later fail completely at a critical time. Moreover, if the display on the tester indicates an erroneous reading, it may not be possible to determine if the problem is in the probe including the probe cable, or in the signal processing circuitry. Furthermore, if the source of the problem is suspected to be in the probe, the tester provides no way of separately testing the probe cable and optics on the one hand, and the signal processing circuitry on the other hand, to isolate the problem area.
Thus, testing the oximeter as an integral entity, i.e., the probe and signal processing circuitry together, as the optical-interface approach requires, does not afford an unqualified independent assessment of these main parts, and especially of the parts most likely to fail. In fact, this testing methodology may introduce other problems as explained above. In contrast, the oximeter testing approach of the present invention differs from optical interface concept described above and, as a result, avoids the resultant problems.